The recently released Snow Leopard 10.6.5 update is not doing anyone any good, according to Ars Technica. While Snow Leopard will still try to connect over IPv6 first, for IPv6 destinations that are reachable over 6to4 it will choose IPv4 instead. In other words, the OS will only connect over 6to4 to IPv6 destinations if there's no IPv4. Apple argues that 6to4 technology is responsible for many non-working IPv6 setups.
In Mac OS X 10.2 Jaguar, Apple introduced IPv6 support which preferred IPv6 over IPv4 if it had a choice. It used 6to4 technology, an automatic tunneling mechanism where IPv6 packets are put inside IPv4 packets. That means users of ISPs that don't support IPv6 yet can still connect to the IPv6 Internet. It looks like Apple has reversed course, and decided to ignore transition technology, opting to wait for an IPv6 world. Hopefully the company will change its decision in a later release.